Yesterday, Apple released the Mac OS X Yosemite 10.10.3 public beta. The 10.10.3 public beta also includes the new Apple Photos app for OS X.
If you are currently enrolled in the public beta seed, you will see the new update appear in your Updates tab.
One word of warning: Photos, just like OS X 10.10.3, is pre-release beta software. That means that there are going to be bugs and you should not install either of these updates on your main, production, Mac that you use daily. To be a beta tester, you must be prepared to delete everything, applications and data (including family photos).
My recommendation is that you make multiple backups before you install OS X 10.10.3 and Photos on your Mac. Time Machine, BackBlaze or event a Finder data copy to an external USB hard disk drive or flash drive is a good idea.
The complete list of updates to OS X Yosemite 10.10.3 updates are as follows:
Mac OS X Yosemite 10.10.3 public beta seed can be installed on any Mac that is currently running OS X Yosemite.